<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      body {
        color: #666;
        text-align: center;
      }
      .box {
        width: 30px;
        height: 30px;
        background: #f40;
        position: fixed;
        left: 0;
        top: 0;
      }
    </style>
    <style>
      /* 在这里完成作业 */
    </style>
  </head>
  <body>
    <div id="ball" class="box"></div>
    <p>在屏幕上点击任意位置试一试</p>
    <p>此效果需要用到JS</p>
  </body>
</html>
